A quadrupole mass spectrometer uses four rod electrodes which are placed parallel to one another and symmetrically in a square array around a center axis (z axis). A DC (direct current) voltage U and a high frequency AC (alternating current) voltage V.multidot.cos.omega.t are applied between a pair of electrodes placed on the x axis and the other pair of electrodes placed on the y axis. When ions are injected at the center of and parallel to the four rod electrodes (that is, along the z axis), ions having a certain mass can go through the space surrounded by the electrodes but ions having other masses disperse from the space. Since the mass of the ions that can go through the space depends on the magnitudes U and V of the DC and AC voltages, mass spectroscopic scanning can be made by changing the values of U and V while maintaining a certain relationship between them. After a scanning is made through masses of a certain range, a mass spectrum curve is obtained having peaks of masses of ions included in the injected ions.
For the proper functioning of the quadrupole mass spectrometer, the dimensions of the four electrodes must be exactly the same and they must be aligned exactly symmetrical. If such conditions are not satisfied, the quadrupole electric field produced by the four electrodes loses symmetry. In this case, peak profiles of the mass spectrum curve would have a long skirt or an irrelevant peak would appear on the skirt, which deteriorates the resolution of mass in mass spectroscopy.